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Am Fam Physician. 2002;65(6):1147-1150

This statement summarizes the current U.S. Preventive Services Task Force (USPSTF) recommendations for screening pregnant women for bacterial vaginosis and the supporting scientific evidence. Explanations of the ratings and the strength of overall evidence are given in Tables 1 and 2, respectively. This is an abridged version of the original Recommendations and Rationale Statement, which appeared in the April 2001 Supplement of the American Journal of Preventive Medicine1 and is available on the Agency for Healthcare Research and Quality (AHRQ) Web site ( The complete information on which this statement is based, including evidence tables and references, is available in the article, “Screening for Bacterial Vaginosis in Pregnancy,”2 and in the Systematic Evidence Review3 on this topic, which can be obtained through the USPSTF Web site (, through the National Guideline Clearinghouse (, or in print through the AHRQ Publications Clearinghouse (800-358-9295).

The USPSTF grades its recommendations according to one of five classifications (A, B, C, D, or I) reflecting the strength of evidence and magnitude of net benefit (benefits minus harms).
A.The USPSTF strongly recommends that clinicians provide [the service] to eligible patients. The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms.
B.The USPSTF recommends that clinicians provide [the service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms.
C.The USPSTF makes no recommendation for or against routine provision of [the service]. The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation.
D.The USPSTF recommends against routinely providing [the service] to asymptomatic patients. The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits.
I.The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service]. Evidence that [the service] is effective is lacking, of poor quality, or conflicting, and the balance of benefits and harms cannot be determined.

Reprints of the original Recommendations and Rationale Statement are available from the AHRQ Web site, through the National Guideline Clearinghouse (, or in print through the AHRQ Publications Clearinghouse (800-358-9295).

The USPSTF grades the quality of the overall evidence for a service on a three-point scale (good, fair, or poor).
Good:Evidence includes consistent results from well-designed, well-conducted studies in representative populations that directly assess effects on health outcomes.
Fair:Evidence is sufficient to determine effects on health outcomes, but the strength of the evidence is limited by the number, quality, or consistency of the individual studies; generalizability to routine practice; or indirect nature of the evidence on health outcomes.
Poor:Evidence is insufficient to assess the effects on health outcomes because of limited number or power of studies, important flaws in their design or conduct, gaps in the chain of evidence, or lack of information on important health outcomes.

Summary of Recommendations

  • The USPSTF concludes that the evidence is insufficient to recommend for or against routinely screening high-risk pregnant women for bacterial vaginosis (I recommendation). (See “Clinical Considerations” for discussion of populations at high risk.)

The USPSTF found good-quality studies with conflicting results that screening and treatment of asymptomatic bacterial vaginosis in high-risk pregnant women reduce the incidence of preterm delivery. The magnitude of benefit exceeded risk in several studies, but the single largest study reported no benefit among high-risk pregnant women.

  • The USPSTF recommends against routinely screening average-risk asymptomatic pregnant women for bacterial vaginosis (D recommendation).

There is good evidence that screening and treatment of bacterial vaginosis in asymptomatic women who are not at high risk do not improve outcomes such as preterm labor or pre-term birth.

Clinical Considerations

  • For women with a history of preterm delivery, screening for bacterial vaginosis is an option.

A single previous episode of preterm delivery by itself may not reliably identify a population of women who will benefit from screening and treatment. Nevertheless, screening may be appropriate in specific circumstances. Studies demonstrating a benefit of screening and treatment were performed among populations of women at especially high risk (35 to 57 percent) of preterm birth. Clinicians should consider previous history of preterm delivery, other risk factors, and time of presentation in making the decision whether or not to screen for bacterial vaginosis in women at high risk.

  • For clinicians electing to screen high-risk women, the optimal screening test is not certain.

Accepted clinical criteria for bacterial vaginosis include vaginal pH level greater than 4.5, amine odor on the application of potassium hydroxide, appearance of a homogeneous vaginal discharge, and presence of clue cells on a microscopic examination of a wet mount. Presence of at least three of these four criteria is generally considered diagnostic of bacterial vaginosis. The use of more limited criteria (e.g., clue cells alone) has not been evaluated.

  • Neither the optimal time to screen high-risk pregnant women nor the optimal treatment regimen for pregnant women with bacterial vaginosis is clear.

The three trials that demonstrated a reduction in preterm birth screened in the second trimester (13 to 24 weeks of pregnancy) used various regimens of oral metronidazole alone or oral metronidazole and erythromycin.

  • Treatment is appropriate for pregnant women with symptomatic bacterial vaginosis infection.

These women were excluded from most screening trials and may be at higher risk than those without symptoms. Treatment can relieve symptoms such as vaginal discharge.

Scientific Evidence


Bacterial vaginosis describes an imbalance in the normal vaginal bacterial flora characterized by a decrease in Lactobacilli and an increase in Gardnerella, Mycoplasma, and anaerobic bacteria. Bacterial vaginosis is a common cause of abnormal vaginal discharge and has been associated with adverse pregnancy outcomes. The true prevalence of bacterial vaginosis in the community is not known. Studies in academic medical centers and public hospitals found that 9 to 23 percent of pregnant women had bacterial vaginosis, with infection more common among African-American women than Caucasian women.

Observational studies have consistently shown an association between bacterial vaginosis and adverse pregnancy outcomes, including preterm delivery (relative risk [RR] ranging from 1.4 to 6.9), preterm premature rupture of membranes (RR = 2.0 to 7.3), spontaneous abortion (RR = 1.3 to 2.0), and preterm labor (RR = 2.0 to 2.6). Preterm delivery is associated with significant respiratory, neurologic, and developmental abnormalities in the newborn that might result in death or long-term disability. A short course of antibiotic therapy can alter the microflora imbalance associated with bacterial vaginosis, but cure rates are variable and recurrences are common. Because bacterial vaginosis may be a marker for adverse pregnancy outcomes, rather than a causative factor, controlled trials have been conducted to determine whether treating bacterial vaginosis will also improve pregnancy outcomes.


The screening test employed in most epidemiologic studies and treatment trials of bacterial vaginosis has been the Gram stain of the vaginal discharge. In practice, a combination of other clinical findings is usually used (see “Clinical Considerations”). Comparisons of clinical criteria and Gram stain yield sensitivities from 62 to 97 percent and specificities from 66 to 95 percent, using the Gram stain as the standard. The use of more limited diagnostic criteria has not been evaluated in studies of adverse pregnancy outcomes.


Seven randomized, controlled trials have evaluated the effect of various antibiotic treatments versus placebo on pregnancy outcomes among women with bacterial vaginosis. Three studies enrolled only high-risk women (primarily those with a history of prior preterm delivery), two reported results separately for women with and without a prior history of preterm delivery, and two enrolled average-risk women. Among four studies reporting results for average-risk women, there were no differences between control groups and treatment groups in rates of preterm delivery, preterm premature rupture of membranes, or delivery of low-birth-weight infants.

Five studies reported conflicting results among women at increased risk because of a history of preterm delivery. Oral antibiotic treatment reduced the incidence of preterm delivery before 37 weeks in three studies, which enrolled women at particularly high risk (incidence of preterm delivery in placebo groups: 35 to 57 percent). In contrast, in a large multicenter, American trial completed in 1999, a different regimen of oral metronidazole provided no benefit for the subgroup of women who had a history of previous preterm delivery. A fifth small study reported no benefit of vaginal clindamycin among high-risk women.


Because bacterial vaginosis is common, screening and treatment could subject a substantial number of women to the inconvenience and minor side effects (primarily nausea) of taking metronidazole and other antibiotics during pregnancy. The regimens used to treat bacterial vaginosis are generally considered safe in pregnancy, but several studies raise the possibility of harms in some women or their infants. In two studies, a subgroup of women who did not have bacterial vaginosis but received treatment with metronidazole or clindamycin experienced trends toward higher incidence of preterm delivery before 34 weeks of gestation (12 to 13 percent versus 4 to 5 percent). In addition, neonatal sepsis was significantly increased among women receiving vaginal clindamycin.


Epidemiologic data and some intervention trials support the hypothesis that screening for and treating bacterial vaginosis may reduce the risk of preterm delivery among some women. Studies published to date do not suggest any benefit of treating bacterial vaginosis among asymptomatic, average-risk women, but additional studies of earlier intervention with different drug regimens are being conducted in these populations. At present, however, the lack of demonstrated benefit and possibility of adverse effects of treatment in women without bacterial vaginosis suggest that routine screening of average-risk women should be discouraged outside of research protocols. For pregnant women with a history of preterm delivery, the inconsistent results of well-done studies prevent a clear recommendation for or against screening. Reasons for the conflicting results are not clear but may involve differences in other risk factors for preterm delivery among enrolled women or differences in drug regimens and timing of therapy. Further studies are needed using diagnostic criteria and treatment protocols that are representative of community practice.

This series is coordinated by Joanna Drowos, DO, contributing editor.

A collection of USPSTF recommendation statements published in AFP is available at

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